The present invention generally relates to a tuner station selecting apparatus, having has at least two local oscillators, such as an up, down tuner station selecting apparatus used, for example, in a CATV system.
Generally, for example, in CATV an American system, since the frequency band thereof ranges from 54 through 440 M Hz, a tuner station selecting apparatus of a double superheterodyne type is normally used. The tuner station selecting apparatus of a double superheterodyne type is also referred to as an up, down tuner station selecting apparatus. Such an apparatus converts a received high-frequency signal into a first intermediate frequency signal by the conversion from the upper side station so as to attenuate the image interference signal, and thereafter, to convert it into a second intermediate frequency signal free from the image interference by the conversion from a lower side station. The basic construction of such a tuner station selecting apparatus is shown in FIG. 1.
The high-frequency signal from an antenna is inputted into a pre-amplifier 1 from an input terminal T1 and is amplified. The amplified high-frequency signal is mixed with a signal from a first local oscillator 6 using a first mixer 2 so as to be converted into a first intermediate frequency signal by the conversion from the upper side station. The first intermediate frequency signal is amplified by a first intermediate frequency amplifier 3, is removed in the image interference signal, and is given to a second mixer 4. The signal is mixed with a signal from the second local oscillator 7 by the second mixer 4 and is converted into a second intermediate frequency signal by the conversion from the lower side station. The signal from the mixer 7 is given to the second intermediate frequency amplifier 5 so as to be amplified, and is guided into an output terminal T2.
The first local oscillator 6 is composed of a voltage control type of oscillator. The output of the local oscillator 6 is divided in frequency by a certain uniform frequency division ratio (for example, 1:64) in the pre-frequency-divider 12, and thereafter, is divided in frequency at a frequency division ratio corresponding to the control signal from the input terminal TC1 with a variable frequency divider 13. The output of the variable frequency divider 13 is given to a phase comparator 10 so as to compare in phase with a reference frequency signal from the reference frequency signal generating circuit 21 composed of an oscillating circuit for crystal oscillator 8 and a reference frequency divider 9. The output of a phase comparator 10 is converted into a control voltage for controlling the local oscillator 6 by a low-pass filter 11. A PLL (phase locked loop) frequency synthesizer is composed of a local oscillator 6, a pre-frequency divider 12, a variable frequency divider 13, a phase comparator 10 and a low-pass filter 11 and so on. The control signal from the input terminal TC1 is changed to change the frequency of the signal from the local oscillator 6 so as to select the high-frequency signal to be converted into the first intermediate frequency signal for turning into the signal of a specific frequency.
For example, the frequency division ratio of the pre-frequency-divider 12 is set at 1:64. The change amount AF of the oscillation frequency of the first local oscillator 7 due to the change of the 1 step of the frequency division ratio in the variable frequency divider 13 at this time is as follows, EQU .DELTA.F=64.times.1(K Hz) (1)
wherein the frequency of the reference frequency signal from the reference frequency signal generating circuit 21 is 1 (K Hz).
Therefore, the high-frequency signal from the input terminal T1 may be received for each 64 (K Hz).
The second local oscillator 7 is composed of a voltage control type oscillator which frequency-discriminates in the second intermediate frequency the signal fed through a limiter 20 from the second intermediate frequency amplifier 5 by a frequency discriminator 19, and is controlled by a control voltage provided through the inputting operation of the output of the frequency discriminator 19 into the low-pass filter 18. An AFC (Automatic Frequency Control) circuit is composed of the limiter 20, a frequency discriminator 19, a low-pass filter 18, a second local oscillator 7 and so on. The stabilization of the second intermediate frequency signal which is guided into the output terminal T2 from the second intermediate frequency amplifier 5 is effected by the AFC circuit. It is to be noted that the block defined by broken lines in FIG. 1 shows a tuner housing member.
In such a prior art as described hereinabove, the operation of the above-described AFC circuit is temporarily interrupted by a construction shown at the switching operation of the station selection for switching the high-frequency signal to be tuned. In such a case, when the oscillation frequency of the second local oscillator 7 has been changed beyond the AFC retracting range due to the changes in the temperature and the humidity, an erroneous operation is caused in the AFC circuit.
Assume the first intermediate frequency is, for example, 965 M Hz. Therefore, when two channels (96 through 103 M Hz) of the Japanese television broadcasting operation are received, the oscillation frequency of the first local oscillator 7 is 1062.25 M Hz. At this time, the sixth higher harmonic of the output signal of the pre-frequency-divider 12 (the frequency divider ratio is 1:64) is caused near 99.58 M Hz. The sixth higher harmonic applies undesirable influences upon the high-frequency signal from the input terminal T1, thus resulting in a deteriorated receiving condition of the two channels. Thus, it is necessary to reduce the influences thereof by the provision of the lower output frequency of the pre-frequency-divider 12 so as to increase the degree of the higher harmonic component which becomes interferences with respect to the received signal. When the frequency division ratio of the pre-frequency-divider 12 is made larger to reduce the influences, the change amount .DELTA.F in the oscillation frequency of the local oscillator 6 with respect to the 1 step of the frequency division ratio of the variable frequency divider 13 becomes larger, so that fine adjustment of the received frequency cannot be effected.